The anthraquinone process for making hydrogen peroxide is well known in the art to be a cyclic process in which 2-alkyl substituted anthraquinones, dissolved in a solvent or mixture of solvents are first hydrogenated in the presence of a hydrogenation catalyst to produce anthrahydroquinones. The anthrahydroquinones are then oxidized, usually with air, to reform the original anthraquinones with concomitant formation of hydrogen peroxide. The hydrogen peroxide is then generally extracted with water and the remaining working solution is dried to remove excess water and is recycled to the hydrogenation step.
During the hydrogenation of anthrahydroquinones, tetrahydroanthraquinones are also produced which in turn are reduced to tetrahydroanthrahydroquinones which form hydrogen peroxide upon oxidation. The term "quinone" is used generically to describe the oxidized state of the alkyl substituted anthraquinones and the alkyl substituted tetrahydroanthraquinones contained in the working solution. The term "hydroquinone" is used generically to describe the reduced state of the working solution.
Hydrogenation catalyst selectivity and activity are considered to be significant factors for obtaining lower cost operation and high utility in the anthraquinone process. U.S. Pat. No. 2,657,980 teaches the advantages of using palladium on activated alumina supports over the prior art of using Raney nickel catalysts. These advantages were attributed to the lower by-product formation rates of palladium and improved resistance of palladium to deactivating effects of hydrogen peroxide. This patent states that suspended catalysts on activated alumina give better results than when a fixed-bed catalyst is used under otherwise comparable conditions. Activated alumina was meant to describe any natural or synthetic hydrated alumina containing alpha alumina monohydrate, gamma alumina, or both. These materials typically have BET surface areas in the range of 100 to 300 square meters per gram (m.sup.2 /g). (BET is a method for measuring the surface area of material as described by Brunauer, Emmett, and Teller in their article "Adsorption of Gases in Multimolecular Layers" in the Journal of the American Chemical Society, Vol. 60, page 309, February 1938, and is well practiced in the art of describing catalysts and supports.)
The extraction efficiency of removal of hydrogen peroxide from working solution is generally less than complete. Extraction equipment is designed and is generally operated in a manner which minimizes the amount of unextracted hydrogen peroxide so as to improve hydrogen peroxide yield.
U.S. Pat. No. 3,887,490 teaches that reintroduction of from 250 to 30,000 milligrams (mg) of hydrogen peroxide with each liter (L) of working solution recycled to the hydrogenation step is beneficial for maintaining the activity of noble metal catalysts deposited on a carrier support such as alumina.
U.S. Pat. Nos. 3,635,841 and 3,615,207 teach the use of palladium deposited on alumina supports which are predominately delta and theta phases having essentially no alpha alumina phases present and BET surface areas in the range of 200 m.sup.2 /g to 20 m.sup.2 /g. These patents teach that these catalysts are particular useful for maintaining activity and selectivity in the case of a fixed-bed, whereas catalysts prepared on predominately alpha alumina exhibit loss of metal and have rather short life times in a fixed-bed.
U.S. Pat. No. 3,488,150 teaches that catalysts containing palladium in admixture with from 0.1 to 50 weight percent of another metal of the platinum group are useful as hydrogenation catalysts for the anthraquinone process to improve hydrogenation selectivity and/or activity. The improvements sought in that work were suppression of ring hydrogenation of the anthraquinones to tetrahydroanthraquinones and octahydroanthraquinones. High concentrations of ring hydrogenated materials are undesirable as their hydrogenated forms are more difficult to oxidize to form hydrogen peroxide and the octahydroanthraquinone species have low solubility in normal solvent mixtures used for the anthraquinone process. The mixed metal catalyst were shown to exhibit improved selectivity for ring hydrogenation when they were intimately mixed as a suspension catalyst of 0.01 to 1.0 micron size, but exhibited no selectivity improvement when deposited on a support such as active alumina oxide, the phase and composition of which were undefined. No advantage for the mixed metal catalysts in fixed-bed operation is cited.
We have found that reintroduction of hydrogen peroxide in concentrations greater than 160 mg/liter produces the formation of acidic products which strongly adhere to the catalyst. The concentration of these acidic products continue to increase with time and/or increasing concentration of hydrogen peroxide. These acidic products cause loss of selectivity of the hydrogenation of the anthraquinones to form undesirable by-products which increase the cost of manufacture and lower the productivity of the system.
The buildup of acidic products eventually shortens the useful life of the catalyst because of low activity and loss of selectivity. The level of acidic products on the catalyst can be controlled by removal of a portion or all of the catalyst periodically and replacement with regenerated or freshly prepared catalyst. Alternatively, the catalyst may be regenerated in place. Also, when the concentration of hydrogen peroxide reintroduced in the working solution which is recycled to the hydrogenator is greater than 400 mg/liter, there is a very rapid formation of acidic products and generation of sufficient quantities of carbon monoxide to cause nearly complete loss of activity, necessitating catalyst replacement or regeneration to restore activity and selectivity.
The source of the acidic products is unknown at this time, but there is evidence that the acidic products result from an oxidation reaction of hydrogen peroxide with organic materials contained in the working solution. Oxalic acid is a major end product of this oxidation process, as it has been identified on the catalysts and its concentration increases with increasing concentration of hydrogen peroxide fed to the catalyst during hydrogenation or with operating time at low concentrations of hydrogen peroxide.
This production of acidic by-products is undesirable for all forms of noble metal catalysts on a support, but is particularly damaging to a fixed-bed catalyst system. Hydrogen peroxide entering a fixed-bed hydrogenator continually contacts the same portion of catalyst at the inlet of the reactor, thereby increasing acidic product concentration on the catalyst in the inlet section more rapidly then elsewhere in the reactor. Also, the formation of carbon monoxide in the inlet section can cause subsequent activity loss in the remaining portion of the bed. In the case of slurry or suspended catalysts, the formation of acidic by-products is generally distributed more uniformly over the entire catalyst and, therefore, increases in concentration at a much lower rate at a given concentration of hydrogen peroxide.
Unexpectedly, it has been found in the present work that catalysts prepared on supports with alpha alumina retain their metal composition, have very low adsorption of acidic products, and retain their activity and selectivity for commercially serviceable times. These catalysts are also more resistant to concentrations of hydrogen peroxide which can cause deactivation of the hydrogenation catalyst and formation of acidic products on the catalyst.